Electron beam concentrating



Nov. 2, 1943. c. v. LrrToN I ELECTRONBEAM CONCENTRATING SYSTEM original4 Filed July 15. 1940 /r//l//l//L/f. M W Y l w 4 m.. s w W M M. y, i.. W W 4. ,r//z/ 3 J n w 3.. b F

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Reissuecl Nov. 2, 19434 Re. .22,389A

ELECTRON BEAM CONCENTRATING SYSTEM Charles V. Litton, Redwood City, Calif., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Original No. 2,305,884, dated December 22, 1942, Serial No. 345,284, July 13, 1940. Application for reissue July 29, 1943, Serial No. 496,626

/ claims. (el. 315-6) This invention relates to systems for concentrating a beam of electrons and more particularly to means for concentrating a beam of electrons in a velocity modulated tube.

'I'here are many systems in which a beam of electrons is used. In such systems it is often desirable that a beam of electrons be concentrated as much as possible. For example, in vacuum t'ube systems operating on the principle of velocity modulation it is desirable that the circuits controlling the velocity modulation and for extraction of energy from the beam be arranged so that no electrons collide with elements in the circuit. However, in order to effectively control the electron beam it is necessary that the controlling electric fields be eiective substantially throughout the cross-sectional area offthe beam.

In` systems previously proposed this is accomplished by means of a grid electrode arranged extending into the area of the beam. This electrode is generally formed of flat metal sheets arranged edgewise to theelectron beam. Even with this precaution it is found that considerable energy ls lost by collision of electrons with the grid and a consequent heating thereof.

If means is provided for concentrating the beam to a very small diameter, then a single opening may be provided in an element of the circuit to control the beam and no grid electrode is required.

It is a principal object of my invention to provide magnetic means for concentrating a beam of electrons.

It is afurther object of my invention to provide a magnetic circuit having a magnetic eld distribution to produce the desired beam concentration.

It is a still further object of my invention to provide a velocity modulated vacuum tube in which grid electrodes are not required.

According to my invention I accomplish the above objects and other objects by providing a magnetic circuit having a held of high intensity converging in the direction of travel of an electron beam. This eld -may be formed by means of electromagnets or permanent magnets arranged so that the lines of magnetic force converge around the beam.

In a velocity modulated tube the beam may be concentrated to a very small diameter. If then the beam is passed successively through resonant cavities to produce velocity modulation, and to extract energy from the beam, only small openings need be provided and no grid in the usual meaning of the term is needed to distribute the electric potential thereacross.

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particular embodiments thereof taken with the accompanying drawing, in which:

Fig. 1 shows an embodiment used to explain the principles of my invention;

Fig. 2 shows a modification of my invention using a preferred distribution of magnetic fields; and

Fig. 3 shows a further modification of my invention using permanent magnetsI to produce the control.

In Fig. 1 is disclosed an embodiment of a velocity modulation tube incorporating one form of magnetic concentration arrangement in accordance with my invention. In this arrangement is provided a cathode IIl for emitting electrons which are formed in a beam by any suitable accelerating means, for example, by application of a positive potential to the other portion of the tube structure such as the resonant cavity. 'I'he beam of electrons then may. pass through resonant chambers I2 and I3. The resonant chamber I2 serves to modify the velocity of the electrons in the beam and may be referred to as the modulation circuit. Electrons so modulated then pass through tube I4 which permits the electrons to become bunched due to their changes in velocity caused in chamber I2. Resonant` chamber I3 then may extract energy from the hunched electrons for application to any suitable circuit not shown. Circuit I3 will hereafter be referred to as thel extractor circuit. The electrons after passing cir- A better understanding of my invention may potential stress from modulation circuit I2 to' the electron stream in order to control the velocity modulation. This is generally accomplished by arranging a grid across the area covered by the stream and connected to circuit I2 so that the potential of circuit I2 will be applied more or less uniformly over the stream. It has been found, however, that even though these grids are designed to be open mesh work, quite a number of electrons strike the'grid and cause excessive heating. Such excessive heating serves as a power limitation to which the tube can operate. Furthermore, in order to achieve high emciency it is desirable that the depth of eld penetration at the resonant circuit represents a small fraction of fy where y is the wavelength in free space and represents the velocity of the electron beam divided by the speed of light.- This result may be obtained either by using a high velocity stream, or by reducing the size of the electrode opening or both.

In accordance with my invention I use no grid at all in connection with circuits I2 and I3 but use small size apertures such as I6, I1 in connection with resonant circuit I2 and in connection with circuit I3. These apertures are made sufciently small so as to preserve substantially uniform control of the electrons in this stream and not permit any great depth of penetration.

In order that the small apertures may be used, I provide in accordance with my invention lan arrangement for concentrating the electrons from a relatively large source such as cathode Ill to produce a beam having an increased density of electrons.

It is known that electrons traveling at any given velocity will tend to follow magnetic lines of force in a spiral path which path generally follows the lines of force. Accordingly, I utilize this principle for the purpose of producing a concentration of the electrons emitted from cathode I0. Thus, I provide a magnetic circuit indicated at 20, having a pole 2| and a second pole 22 both preferably, but not `necessarily, of annular form. Pole 2I is placed outside cathode III, while pole 22 is arranged with an'opening of small diameter. Accordingly, a substantially frusto-conical field of magnetic force is set -up between poles 2I and 22,- as shown by dotted lines in Fig. 1. Electrons emitted from cathode IU are thus concentrated by the magnetic field into a small beam. In order that the full efliciency of the system be retained, however, it is desired that a further concentration of the electron beam be made toassure that the beam is sufciently small to pass through openings I6, I1, I8 and I9. In accordance with my invention this may be accomplished by an additional magnetic circuit 30, which is provided with pole pieces 3l, 32. Pole piece 3I is ai. annulus of relatively large diameter, while pole piece 32 is an annulus of smaller diameter. The lattermay be arranged inside the actual tube structure itself. This may serve to spread the electron beam outwardly. It is clear that other means may be used for spreading the electron beam, such as a transverse magnetic circuit, or a circuit having diverging magnetic lines, or a positive ring electrode tending to attract the electrons outwardly thereto.

In an actual trial of concentrating arrangements in accordance with my invention with a beam of electrons accelerated by a 3000 volt potential and by use of a eld of several hundred gauss, substantially all of the electrons from a 5/8 diameter cathode were concentrated into a beam of 2 mm. in diameter. This concentration was done in a beam traveling a distance of 5", and having an intensity of 200 milliamperes, that is, an average Adensity of about 64 milliamperes per square millimeter. It is clear that, while I have illustrated an application of my concentrating system to a velocity modulated tube arrangement, other applications of this arrangement may be found useful. It is clear that the principles of my invention with respect to the concentration yof electron beams, while constituting a substantially essential combination in so far as the tube operation is concerned, may have other elds of application independent of the velocity modulated tube arrangement.

While I have shown in Fig. 1 an embodiment ofl my invention for the purpose of explaining the operation thereof, it is clear that in some instances a circuit such as shown therein will be insuiiicient to produce the desired concentration.

be done by plating soft iron material with suitable plating material and welding or brazing it to the non-magnetic tube envelope material preferably copper. The small gap caused by the intervention of the copper layer in the magnetic circuit 3D may be of such small dimensions as to be unimportant with respect to the larger air gaps between pole pieces 3I and 32.

In order to secure relatively great concentration it is necessary that the magnetic iield produced by magnetic circuits 20 and 3l) be relatively strong. Accordingly, it is suitable to utilize velectro-magnet for this purpose and to' this end I provide magnetizing coils 23 and 33 associated with magnetic circuits 20 and 30, respectively.

. It is clear that if suitable magnetic concentra- The concentrated beam of electrons after passing through apertures I8, I9, is conducted to an enclosurel 36. However, the .beam may be so highly concentrated that it is desirable to spread the beam somewhat .before it' is permitted to come in contact with the target electrode I5.

This may be accomplished, for example, by provision of a conical shaped electrode 31 upon which is applied a negative potential which will This is particularly true of the magnetic circuit related to the resonant chambers of the tube, since the air gap in this system may be so great that it is diiiicult to secure suflicient concentration of magnet elds at the point adjacent openings I6, I'I, I8 and I5. In Fig. 2 is shown a modification of my invention, wherein separate magnetic circuits are provided about each of 'the resonant chambers of a velocity modulating tube. In this disclosure is shown a similar form of velocity modulating tube having a cathode III, an accelerating electrode II, resonant chambers I2, I3, with associated control openings I6, Il and I8, I9, respectively and a target electrode I5. About the cathode end of the arrangement is provided a iirst magnetic circuit 20 with pole pieces 2l, 22. Pole piece 22 is shownas arranged in the tube envelope structure and separated from Vmagnetic circuit 20 by means of the thickness ofl the material therein. It is clear, however, that if desired this pole piece 22 may be sealed Within the envelope, in the manner indicated in Fig. 1. When a sealing of ythis soft iron metal with the copper is provided in atube 'structure it is desired that the proper materials be corrugated or modiiied in some manner so that unequal expansion of the two metals will n ot cause a' break in the envelope destroying the vacuum.

Associated with resonant chamber I2, I provide a second magnetic circuit 3ll,pole pieces 3|, 32. Thus the air gap across apertures I6, I1 is relatively small and a strong magnetic concentrating field may therefore be produced so as more eiectiveiy to stop down, narrow, or focus the beam as it passes @through these openings.

Thus a third magnetic circuit 40 is provided 'about resonant circuit I3. provided with a pair of pole, pieces 4I, 42, which provide a narrow air gap across openings I8, I9

so as to concentrate the beam while passing therethrough. I

Individual energizing coils 23, 33 and 43,` are This arrangement is sasso provided for magnetic circuits I0, 30 and l0, respectively. These energizing coils should be so energized that the polarity of each magnetic field is properly related one to another to produce the desired conical concentration and not to furnish an interference one with another.

In this arrangement I have disclosed a ring electrode 30 insulated from the wall of enclosure 30, by a strip of insulation 30. This electrode 30 is raised to a small potential so as to spread the electron beam prior to its impingement upon target electrode I5.

In Fig. 3 I have illustrated a third embodiment of my invention. This embodiment is similar to Fig. 2 in that it is a system for providing individ, ual magnetic circuits forMA the cathode beam arrangement and for each of the resonant chambers I2. I3. However, instead of using electromagnets, the arrangement shown in Fig. 3, provides permanent magnets for producing the concentrating field. Accordingly, the magnetic circuit 30 for concentrating the electron stream as emitted from a cathode comprises a permanent magnet 303 which maybe made in the form of a tubular ring. Soit iron pole pieces 30|, 302 are provided for producing the concentrating eld and these pole pieces are connected with the opposite ends of magnets 303 by means oi' soft iron rings or spiders 305, 306. 'I'he magnetic circuit 30 about resonant chamber I2 comprises a permanent magnet shaving pole pieces 40I, 402 and spiders 405, 406 interconnecting the permanent magnets and the soit iron pole pieces. In order that the magnetic circuits may be lprop-- erly operated and will not interfere with one another it is necessary that magnets 303 and 403, be arranged with like poles toward one another. A third magnetic circuit is provided around resonant chamber I3. This comprises a permanent magnet E03 with soft iron pole pieces 50|, 502 and interconnecting spiders.i05, 500. Pole pieces 502 ii desired may be extended axially outward from magnet 503 and may serve as a further pole pie for a fourth magnetic circuit herein provided.

A fourth magnetic circuit is provided for the purpose of spreading the electron beam after it has passed through resonant chamber I3. This arrangement comprises a permanent magnet 603, associated by means of a spider or disc arrangement 005 with the extended portion of pole piece 502. Another pole piece which may be a soft iron ring 000 is provided at the opposite end of permanent magnet 003. It will be noted that the air gap between 000 and 502 is such as to provide a diverging magnetic iield. Accordingly, the transverse component of this iield will tend to spread the electron beam so that upon impingement on electrode Il the energy will not be too highly concentrated.

While I have disclosed certain features of my 'invention independently in Figs. 1, 2 and 3, it should be distinctly understood that this is done for the purpose of simplifying the showing. It is clear that the various features of each of these arrangements which properly apply may be readily exchanged ior systems shown in the other arrangements, or may be used in addition thereto. For example, if desired any one of the systems tor spreading the electron beam or any combination of these systems may be used ina singie tube structure. Furthermore, it should be realised that the magnetic structures shown are 3 merely illustrative of the invention and that any modiiication in shape and arrangement of this structure may be made within the spirit of my invention. What I consider to be my invention and upon which I desire protection isembodied in the accompanying claims.

what I Claim SZ s l. A vacuum tube comprising means' for producing a stream of electrons, means for modifying said electron stream, means for extracting energy from said modied electron stream, and a magnetic circuit including a relatively short air gap, said air gap extending along said stream Afor producing substantiallyconverging lines of magnetic force in the direction of travel of the electrons of said stream to concentrate said stream inta a small cross-sectional area, said means for modifying said electrons comprising a resonant chamber including a pair of grids having a single opening in the path of the electron stream, and-wherein said magnetic circuit is generally coaxially disposed about said electron stream and comprises a first part between said electron producing means and said opening for producingY converging lines of force to concentrate said beams before reaching said opening,

electrons have passed said means for extracting I y energy therefrom, and magnetic means for diffusing the electrons oi said beam before striking said target.

4. An electronic discharge device comprising means for producing a stream of electrons, means forming a resonant chamber longitudinally spaced from said electron producing means and in the path of said stream, said resonant chamber forming means including a stream-controlling element havinga small opening in the direct path of the electron stream, said opening having a cross-sectional area substantially smaller than the initial cross-sectional area of the produced stream, and means for concentrating the produced electron stream into a beam having a cromsectional area substantially less than that of said opening and uniform over a substantial portion of its length. e

5.*The combination according to claim 4, in which said last-means comprises a pair of annularmagneticpolepieceseaehencompassinga substantially greater cross-sectional area than the opening in said control element, one of said pole pieces being positioned about and adjacent the electron. stroamproducing means, and the other encompassing a cross-sectional area lessthan that of the said one pole piece. podtiionedl about the -electron stream intermediate the' electron stream produdng means and said control element. f

CHARLES V. LITI'ON. 

